The invention is directed to amplifiers of the Pulse width modulated (PWM) type and more particularly to improved long life Metal-Oxide Semiconductor Field-effect transistors (MOSFET) used in class D Amplifiers.
Prior art PWM amplifiers such as, amplifier 10, shown in the prior art FIG. 1, the MOSFETs (12A and 12B) are found to have a very short operational life due to an inherent internal clamping diode (14A and 14B) incorporated in the construction of a MOSFET. The MOSFET internal clamp diode is connected from source to drain as shown. The typical MOSFETs shown are IRF 240 devices or equivalent. The amplifier input, MOSFET elements and ground connections, capacitors C1, C2 and C3, inductance L1 and speaker 16 are conventional components of a typical PJM output stage. The internal clamp diode causes a problem in that it is to slow to efficiently accommodate desirable PWM switching speeds for a stereo audio amplifier which require switching rates in the range of 400 khz.
The problem caused by the internal clamping diode becomes apparent when very fast rise times are used to switch between the pair of MOSFET devices to obtain efficient stereo audio amplification. Typical switching rates for quality audio power applications are several hundred kilohertz (KHZ) and above. Switch rise times in the order of 20 nanoseconds (NS) are required to provide the necessary efficiencies for stereo audio operation. The clamp diode has the characteristic of a relatively slow decay time typically about 600 NS. When there is no audio signal applied to the amplifier, the current through inductor L1 is a symmetrical triangular waveform with an average low frequency component of zero. As the audio signal is increased the current wave form through inductor L1 becomes unsymmetrical in proportion to the audio wave form. The current flowing in the output coil (L1) is a composite of the audio current, and the inductor L1 magnetizing current. When this composite current minimum is greater than zero, current is still flowing in the diode 14B when MOSFET 12A switches on or current is still flowing in diode 14A when MOSFET 12B switches on. The large reverse recovery current is destructive to the MOSFETs and is additionally destructive because of the high peak reverse voltage spikes present.
The current into inductor L1 will build up until the minimum instantaneous composite current will always exceed 0 amps.
During the switched off part of the cycle when Vb is more minus than the B- supply, current will flow back into capacitor C2 through diode 14B. This current will continue to increase the voltage on capacitor C2 until the composite current waveform has approached 0 amps.
The result of this current (particularly at low modulation frequencies) causes a power supply unbalance by increasing the magnitude of B-. This causes a distortion in the output audio wave form and additionally can create a destructive voltage across capacitor C2. The same effect occurs when negative current is flowing into the load, causing B+ to rise in magnitude.
There has not been a successful MOSFET class D audio amplifier or a successful solution to the power supply unbalance created by a class D MOSFET audio amplifier until the emergence of the instant invention.